1. Field of the Invention
The present invention relates to an image information signal transmission apparatus for transmitting an image information signal.
2. Related Background Art
A conventional method of reducing a sampling frequency and another conventional method of reducing the number of average bits of sample data per sample are known as methods of reducing a transmission bandwidth of an image information signal such as a television (TV) signal. According to the former method, a subsampling point obtained by reducing the
number of samples into 1/2 by subsampling and a flag representing a position of another subsampling point used in interpolation (this flag is a positional information data representing that sample data of any one of the four adjacent subsampling points with respect to an interpolation point is used) are transmitted.
According to one of the examples of the latter method, a one-field image is divided into small blocks, and sample data within each block is encoded. This method is known as a block coding method. According to the block coding method, for example, a difference between the minimum (MIN) and maximum (MAX) values of the sample data within any block is quantized linearly or nonlinearly, and index data which represents a quantization level to which the quantized value belongs in units of samples is transmitted, and the MAX and MIN value data are transmitted as scale components.
According to this conventional block coding method, sample data within each block is coded in correspondence with its dynamic range. For example, if sample data having an extremely large level difference is present in the block, or sample data representing a boundary between the images is present in the block, i.e., if a sample data distribution is greatly different from a preset distribution, the decoded image information signal is greatly degraded.
When the dynamic range of the sample data within the block is quantized to suppress the degradation of the image during decoding, the number of quantization levels or the number of quantization bits can be increased. However, the volume of transmission information is increased, and a good data compression effect cannot be expected.